Solid-state imaging device

ABSTRACT

A solid-state imaging device includes a photoelectric conversion section which is provided for each pixel and which converts light incident on a first surface of a substrate into signal charges, a circuit region which reads signal charges accumulated by the photoelectric conversion section, a multilayer film including an insulating film and a wiring film, the multilayer film being disposed on a second surface of the substrate opposite to the first surface, and a transmission-preventing film disposed at least between the wiring film in the multilayer film and the substrate.

CROSS REFERENCES TO RELATED APPLICATIONS

The subject matter of application Ser. No. 11/253,823 is incorporatedherein by reference. The present application is a divisional of U.S.application Ser. No. 11/253,823, filed Oct. 19, 2005, which claimspriority to Japanese Patent Application No. JP2004-306182, filed Oct.20, 2004. The present application claims priority to these previouslyfiled applications.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a backside-illumination-typesolid-state imaging device in which a light-receiving surface of aphotoelectric conversion section is disposed on a backside of asubstrate. In particular, the invention relates to a solid-state imagingdevice that is suitable for use in CMOS image sensors manufactured usinga MOS process.

2. Description of the Related Art

Recently, in solid-state imaging devices, such as CCD image sensors andCMOS image sensors, with the reduction in the size of devices to bemounted, the chip area has been reduced and reduction in the area perpixel has been necessitated. As a result, in a solid-state imagingdevice which receives light from the front side of a chip substrate,where electrodes and lines are placed, light is blocked by theelectrodes and lines, and thus it is not possible to have satisfactorylight collection characteristics. As a technique to overcome thisproblem, a backside-illumination-type solid-state imaging device isfabricated in which light is received from the backside of a substrate,where no lines or electrodes are placed, and photoelectric conversion isperformed in the substrate to improve light collection characteristics.For example, refer to Japanese Unexamined Patent Application PublicationNo. 2003-338615 (Patent Document 1).

FIG. 1 is a cross-sectional view showing an example of a typicalstructure of a backside-illumination-type solid-state imaging device(CMOS image sensor) in the past.

In the solid-state imaging device, light-receiving portions 12 ofphotoelectric conversion sections (photodiodes) and element isolationregions 14 are disposed in a semiconductor substrate 10, and gate oxidefilms 16 of MOS transistors, gate electrodes 18, contacts (not shown),wiring films 22 and 26, interlayer insulating films 24 and 28, aninsulation covering film 30, etc. are disposed on and above thesemiconductor substrate 10.

In such a backside-illumination-type device, light is received from thebackside, and light incident on the light-receiving portion 12 in thesemiconductor substrate 10 is subjected to photoelectric conversion.Light received from the backside is not entirely subjected tophotoelectric conversion in the substrate 10, and as indicated by anarrow A, a part of incident light reaches the front surface of thesubstrate 10 and is further transmitted through the upper layers abovethe front surface of the substrate 10. The light transmitted through theupper layers is reflected by the wiring film 26 composed of a metallocated in the upper layer above the substrate 10. As indicated by anarrow B, some part of the reflected light enters the photoelectricconversion section again.

When such reflected light returns to and enters the photoelectricconversion section through which the light has been originallytransmitted, substantially no adverse effect is caused. However, whenthe reflected light enters another photoelectric conversion sectionother than the original photoelectric conversion section, colorseparation characteristics may be degraded, and flare or the like mayoccur.

Under these circumstances, a technique has been proposed in which a gateelectrode or a gate oxide film is allowed to function as a metalreflection film or a dielectric reflection film to prevent the adverseeffect of reflected light.

SUMMARY OF THE INVENTION

It is, however, difficult to allow a gate oxide film to function as areflection film using a MOS process. Such an approach is not disclosedin Patent Document 1 and is not feasible.

Furthermore, although it is possible to allow a gate electrode tofunction as a reflection film, such an approach alone is not sufficient.For example, as shown in FIG. 1, if light passes through the photodiode,light reflected by the wiring film located in the upper layer is highlylikely to reenter another pixel.

There is a need for a backside-illumination-type solid-state imagingdevice which can prevent light that has entered from the backside of asubstrate, has been transmitted through a photoelectric conversionsection, and has been reflected by a wiring film from improperlyreentering another photoelectric conversion section and in which ahigh-quality image can be obtained.

According to an embodiment of the present invention, a solid-stateimaging device includes a photoelectric conversion section which isprovided for each pixel and which converts light incident on a firstsurface of a substrate into signal charges, a circuit region which readssignal charges accumulated by the photoelectric conversion section, amultilayer film including an insulating film and a wiring film, themultilayer film being disposed on a second surface of the substrateopposite to the first surface, and a transmission-preventing filmdisposed at least between the wiring film in the multilayer film and thesubstrate.

According to another embodiment of the present invention, a solid-stateimaging device includes a photoelectric conversion section which isprovided for each pixel and which converts light incident on a firstsurface of a substrate into signal charges, a multilayer film includingan insulating film and a wiring film, the multilayer film being disposedon a second surface of the substrate opposite to the first surface, anda transmission-preventing film which is disposed above the secondsurface of the substrate and at a position closer to the substrate thanthe wiring film.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view showing an example of a structure of asolid-state imaging device in the past;

FIG. 2 is a cross-sectional view showing an example of a structure of asolid-state imaging device according to Example 1 of the presentinvention;

FIG. 3 is a circuit diagram showing an example of a configuration of apixel transistor circuit in the solid-state imaging device shown in FIG.2;

FIG. 4 is a cross-sectional view showing a specific example of a methodfor fabricating the solid-state imaging device shown in FIG. 2;

FIG. 5 is a cross-sectional view showing a specific example of a methodfor fabricating the solid-state imaging device shown in FIG. 2;

FIG. 6 is a cross-sectional view showing a specific example of a methodfor fabricating the solid-state imaging device shown in FIG. 2;

FIG. 7 is a cross-sectional view showing a specific example of a methodfor fabricating the solid-state imaging device shown in FIG. 2;

FIG. 8 is a cross-sectional view showing a specific example of a methodfor fabricating the solid-state imaging device shown in FIG. 2;

FIG. 9 is a cross-sectional view showing a specific example of a methodfor fabricating the solid-state imaging device shown in FIG. 2;

FIG. 10 is a cross-sectional view showing a specific example of a methodfor fabricating the solid-state imaging device shown in FIG. 2;

FIG. 11 is a cross-sectional view showing a specific example of a methodfor fabricating the solid-state imaging device shown in FIG. 2;

FIG. 12 is a cross-sectional view showing a specific example of a methodfor fabricating the solid-state imaging device shown in FIG. 2;

FIG. 13 is a cross-sectional view showing an example of a structure of asolid-state imaging device according to Example 2 of the presentinvention;

FIG. 14 is a cross-sectional view showing an example of a structure of asolid-state imaging device according to Example 3 of the presentinvention; and

FIG. 15 is a block diagram of the present invention applied to asolid-state imaging apparatus.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

According to an embodiment of the present invention, in a CMOSsolid-state imaging device of a backside-illumination type, atransmission-preventing film is provided between a silicon substrate anda wiring film in the lowest layer of a multilayer film, thetransmission-preventing film preventing light that has entered from thebackside of the silicon substrate and has been transmitted through thesilicon substrate from reaching the wiring film. Thereby, it is possibleto prevent light reflected by the wiring film from improperly enteringanother pixel and being subjected to photoelectric conversion.Specifically, a silicide film may be used, the silicide film beingformed by placing a cobalt film or the like on the surface of a siliconsubstrate and the surface of a polysilicon electrode film and performingheat treatment. Alternatively, a dummy metal wiring film serving as areflection film may be provided in the lowest layer of the multilayerfilm, or a light-absorbing film may be used as an interlayer insulatingfilm.

Example 1

FIG. 2 is a cross-sectional view showing an example of a structure of asolid-state imaging device according to Example 1 of the presentinvention, and FIG. 3 is a circuit diagram showing an example of aconfiguration of a circuit region (pixel transistor circuit) in thesolid-state imaging device shown in FIG. 2. FIGS. 4 to 12 arecross-sectional views each showing a specific example of a method forfabricating the solid-state imaging device shown in FIG. 2.

First, as shown in FIG. 3, in the solid-state imaging device accordingto this example, each pixel is provided with a photodiode (PD) 100functioning as a photoelectric conversion section, a reset transistor110 for resetting charges accumulated in the photodiode 100, anamplifier transistor 120 for amplifying and outputting pixel signalscorresponding to the amount of accumulated charges, a transfertransistor 130 for selecting the timing of transfer of signal chargesaccumulated in the photodiode 100 to the gate of the amplifiertransistor 120, and a selection transistor 140 for selecting a pixelfrom which signal charges are read. Additionally, such a configurationof pixel circuit may be the same as that used in the past, or anothercircuit configuration, for example, a configuration which does notinclude a transfer transistor, may be used.

Referring to FIG. 2, a photoelectric conversion section (photodiode) andthe MOS transistors described above are disposed in a region isolated byan element isolation region 214 in a semiconductor substrate 210 as abase member. A light-receiving portion 212 of the photodiode receiveslight entering from the backside of the semiconductor substrate 210 andperforms photoelectric conversion. In this example, the photodiode has ahole accumulation diode (HAD) structure in which a p-type layerfunctioning as a hole accumulation region is provided on alight-receiving surface of an n-type layer functioning as an electronaccumulation region. A transfer transistor is disposed adjacent to thephotodiode, and a gate electrode 218 is disposed on the substrate with agate oxide film 216 therebetween. Signal charges generated in thephotodiode are transferred to a floating diffusion section (drain) 220.

Furthermore, a silicide film 222 (CoSi₂ or the like) is formed on theupper surfaces of the gate oxide film 216 and the gate electrode 218,the silicide film 222 being produced by alloying a silicon layer with ametal, such as cobalt. Contacts (not shown), wiring films 226 and 230,interlayer insulating films 228 and 232, an insulation covering film234, etc. are further provided thereon.

In such a solid-state imaging device, since the silicide film 222functioning as a transmission-preventing film is disposed between theupper surface (front surface) of the semiconductor substrate 210 and thewiring films 226 and 230, light that has entered from the backside ofthe semiconductor substrate 210 and that is transmitted through thephotoelectric conversion section is reflected by the silicide film 222toward the photoelectric conversion section. Thus, it is possible toprevent transmission of light to the wiring films 226 and 230 andreflection of light from the wiring films 226 and 230.

A method for forming a silicide film as that shown in FIG. 2 will now bedescribed with reference to FIGS. 4 to 12.

First, referring to FIG. 4, a p-type well layer 320 is formed in aregion for forming a photodiode section and a floating diffusion (FD)section in an n-type silicon substrate 310. An element isolation region330, a gate oxide film 340, and a gate electrode (polysilicon film) 350are formed on the upper surface of the silicon substrate 310. Then,pattering of a photoresist layer 360 is performed, and n-type ions areimplanted into the photodiode section to form an n-type layer 370 of thephotodiode.

As shown in FIG. 5, patterning of a photoresist layer 380 is performed,and a thin n-type layer 390 is formed in the FD section by implantationof ions. Furthermore, as shown in FIG. 6, an oxide film 400 is depositedover the entire surface. Then, as shown in FIG. 7, an etch-back processis performed to form a sidewall 410.

Subsequently, as shown in FIG. 8, patterning of a photoresist layer 480is performed, and ions of high density are implanted into the FD sectionto form an LDD structure. As shown in FIG. 9, patterning of aphotoresist layer 490 is performed, and p-type ions are implantedshallowly into the photodiode section. A photodiode with a HAD structureis thereby produced.

Next, as shown in FIG. 10, a Co film 430 is formed by sputtering overthe entire surface. By subsequent annealing, as shown in FIG. 11, aCoSi₂ film 440 is formed at a portion in which the Co film 430 and Si ofthe silicon substrate 310 are in contact with each other and at aportion in which the Co film 430 and poly-Si of the gate electrode 350are in contact with each other. Furthermore, at a portion in which theCo film 430 and SiO₂ of the gate oxide film 340 are in contact with eachother, CoSi₂ is not formed. Thus, the silicide films 440 are formed inthe Si and poly-Si regions. Then, as shown in FIG. 12, wet etching isperformed to remove the Co film on the gate oxide layer 340.Subsequently, the same steps are carried out as those in the past tocomplete a solid-state imaging device, which is not directly related tothe present invention. Therefore, a description thereof will be omitted.

Thus, silicide films are formed on the surfaces of the photodiodesections and the source/drain sections. As indicated by an arrow C inFIG. 2, it is possible to block light entering from the backside of thesubstrate. Consequently, light transmitted through the silicon substrateis not reflected by the wiring films located in the upper layers and isnot subjected to photoelectric conversion in a light-receiving portionof another pixel. Thereby, it is possible to prevent degradation incolor separation characteristics or occurrence of flare. Additionally,this Example 1 is merely an example, and the silicide film used in thepresent invention is of course not limited to the cobalt silicide filmdescribed above and may be a tungsten silicide film or the like.Furthermore, the position and the formation method of the silicide filmare not limited to the example described above.

Example 2

FIG. 13 is a cross-sectional view showing an example of a structure of asolid-state imaging device according to Example 2 of the presentinvention. The same reference numerals are used to designate the sameelements as those shown in FIG. 2.

In Example 2, in place of the silicide films described above,light-shielding dummy wiring films 500 are disposed as films thatprevent transmission of light.

As shown in FIG. 13, the light-shielding dummy wiring films 500 aredisposed in the lowest layer under the layers in which wiring filmshaving the original function are disposed. An interlayer insulating film501 is disposed thereunder. The dummy wiring films 500 are not composedof a material that transmits light, such as polysilicon, but arecomposed of a metal, such as tungsten or aluminum, or composed of analloy. As shown in FIG. 13, the dummy wiring films 500 are formed in apattern corresponding to light-receiving regions of the photodiodes. InExample 2, as indicated by an arrow D, light that has entered from thebackside of the substrate and has been transmitted through the substrateis reflected at the position close to the photodiode, and improper entryof light into another pixel can be prevented. Additionally, as thematerial for the light-shielding dummy wiring films, tungsten silicideor cobalt silicide described above may be used.

Example 3

FIG. 14 is a cross-sectional view showing an example of a structure of asolid-state imaging device according to Example 3 of the presentinvention. The same reference numerals are used to designate the sameelements as those shown in FIG. 2.

In Example 3, as films that prevent transmission of light, interlayerfilms having a light-absorbing property are used.

As shown in FIG. 14, interlayer insulating films 510 and 520 aredisposed between the silicon substrate and a first wiring film layer andbetween the first wiring film layer and a second wiring film layer,respectively. These interlayer insulating films 510 and 520 serve aslight-absorbing films. Specifically, for example, silicon carbide (SiC)may be used.

By providing such interlayer insulating films 510 and 520, as indicatedby an arrow E in FIG. 14, even if light that has entered from thebackside reaches the front side of the substrate, light is absorbed bythe interlayer insulating films 510 and 520, and improper entry of lightinto another pixel can be prevented.

In Examples 1 to 3, the transmission-preventing film is mainly providedbetween the substrate and the wiring film in the lowest layer. However,it is to be understood that even when a transmission-preventing film isdisposed in a layer above the wiring film in the lowest layer, asatisfactory effect can be obtained. For example, atransmission-preventing film may be provided between the lowest layerand a wiring film in the second lowest layer.

It should be understood by those skilled in the art that variousmodifications, combinations, sub-combinations and alterations may occurdepending on design requirements and other factors insofar as they arewithin the scope of the appended claims or the equivalents thereof.

Also, the present invention may be applied to a solid-state imagingapparatus, like a camera or a camera module, as is described in FIG. 15.The solid-state imaging apparatus may include a signal processingportion 601 which processes image signals based on the signal chargesand an optical system 602 which conducts incident light to an imagingarea. In that case, the solid-state imaging apparatus can capture a goodquality image.

According to the embodiment of the present invention, in a solid-stateimaging device (or apparatus) of a backside-illumination-type in which alight-receiving surface of a photoelectric conversion section isdisposed on a first surface (backside) of a substrate, atransmission-preventing film which prevents the transmission of light isdisposed between a wiring film located above the substrate and thesubstrate. Thereby, it is possible to prevent light that has enteredfrom the backside of the substrate and has been transmitted through thesubstrate from being reflected by the wiring film and improperlyentering a photoelectric conversion section of another pixel.Consequently, degradation of image quality can be prevented.

Furthermore, the circuit region of the solid-state imaging device mayinclude an amplifier transistor, a selection transistor, and a resettransistor, and may further include a transfer transistor. Degradationof image quality caused by improper reflected light can be preventedwithout being restricted by the structure of the circuit region. Thus,the solid-state imaging device can be easily applied to various pixelstructures.

Furthermore, the photoelectric conversion section may have a holeaccumulation diode (HAD) structure including an n-type layer and ap-type layer. Thereby, higher image quality can be obtained.

Furthermore, the transmission-preventing film may be a silicide film.Thereby, an appropriate transmission prevention function (reflectionfilm) can be obtained. Moreover, if the silicide film is formed byplacing a metal film on a silicon substrate and a gate electrodecomposed of polysilicon and performing heat treatment, it is possible toeasily form the transmission-preventing film in optimal regions.

Furthermore, the transmission-preventing film may be a light-shieldingmetal film or alloy film. Thereby, an appropriate transmissionprevention function (reflection film) can be obtained. Moreover, if thelight-shielding metal film or alloy film is formed in a patterncorresponding to light-receiving regions of the photoelectric conversionsections, the metal film or alloy film can be disposed only in necessaryregions, and thus transmission of light can be effectively prevented.

Furthermore, the transmission-preventing film may be an interlayer filmthat absorbs light. Thereby, an appropriate transmission preventionfunction (light absorption function) can be obtained, and degradation ofimage quality caused by improper reflection can be prevented.

1. A solid-state imaging device comprising: a photoelectric conversionsection which is provided for each pixel and which converts lightincident thereon into a signal; a circuit region which transfers thesignal from the photoelectric conversion section; a multilayer filmincluding an insulating film and a wiring film, the multilayer filmbeing disposed beneath the photoelectric conversion section; and whereinthere are a plurality of multilayer films and each multilayer filmincludes a layer of light absorbing material as the insulating film,wherein the layer of light absorbing material is located at a side ofthe photoelectric conversion section opposite a side at which light isprimarily incident thereon, the light absorbing material absorbing agreater amount of light than silicon dioxide, wherein a gate electrodemade of a polysilicon film with a gate insulating film between thepolysilicon film and a silicon substrate, and a silicon carbide layer isformed adjacent the silicon substrate and the polysilicon film as thelayer of light absorbing material.
 2. The solid-state imaging deviceaccording to claim 1, wherein the circuit region includes an amplifiertransistor for amplifying and outputting pixel signals corresponding tothe amount of accumulated charges in the photoelectric conversionsection, a selection transistor for selecting a pixel from which signalcharges are read, and a reset transistor for resetting chargesaccumulated in the photoelectric conversion section.
 3. The solid-stateimaging device according to claim 2, wherein the circuit region furtherincludes a transfer transistor for selecting the timing of transfer ofsignal charges accumulated in the photoelectric conversion section tothe amplifier transistor.
 4. The solid-state imaging device according toclaim 1, wherein the photoelectric conversion section includes an n-typelayer which accumulates electrons serving as signal charges, and ap-type layer which is provided in a surface region of the n-type layerand which accumulates holes.
 5. The solid-state imaging device accordingto claim 1, wherein the transmission-preventing film is an interlayerfilm that absorbs light.
 6. A solid-state imaging device comprising: aphotoelectric conversion section which is provided for each pixel andwhich converts light incident into a signal; a multilayer film includingan insulating film and wirings, the multilayer film being disposedbeneath the photoelectric conversion section; and wherein there are aplurality of multilayer films and each multilayer film includes a layerof light absorbing material as the insulating film, wherein the layer oflight absorbing material is located at a side of the photoelectricconversion section opposite a side at which light is primarily incidentthereon, the light absorbing material absorbing a greater amount oflight than silicon dioxide, wherein the light absorbing material issilicon carbide.
 7. A solid-state imaging apparatus comprising: aphotoelectric conversion section which is provided for each pixel andwhich converts light incident thereon into a signal; a multilayer filmincluding an insulating film and a wiring, the multilayer film beingdisposed beneath the photoelectric conversion section; wherein there area plurality of multilayer films and each multilayer film includes alayer of light absorbing material as the insulating film, the lightabsorbing material absorbing a greater amount of light than silicondioxide; and a signal processing portion which processes image signalsbased on the signals generated by the photoelectric conversion section,wherein the layer of light absorbing material is located at a side ofthe photoelectric conversion section opposite a side at which light isprimarily incident thereon, wherein the light absorbing material issilicon carbide.